1. Field of the Invention
The present invention relates to an explosive composition and, more specifically, to an explosive composition having reduced slow cook-off violence.
2. State of the Art
Composition B (“Comp B”) is an explosive composition composed of trinitrotoluene (“TNT”) (39.5%), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (“RDX”) (59.5%) and wax (1.0%). For decades, Comp B has been the workhorse explosive for bomb fills, grenades, and anti-personnel mines. Comp B's extensive use is, in part, a result of its physical properties, which are suitable for low-cost, safe manufacturing of ordnance. The excellent chemical and thermal stability of the Comp B ingredients assure that long life-cycles are achievable over a wide range of environmental conditions. Most importantly, Comp B's explosive performance is suitable for a broad range of ordnance applications.
Despite Comp B's versatility and extensive history, considerable improvements in quality, cost, and hazard characteristics have been achieved. Environmental concerns with the manufacture of TNT and its byproducts have eliminated domestic sources of TNT. Also, modern safety, hazards and sensitivity standards require that explosive ordnance exhibit reduced vulnerability and hazards sensitivity. High performance replacements for Comp B have been developed that have reduced hazard sensitivity and are produced using low cost and commercially available ingredients, preferably nontoxic omoncarcinogenic. One such replacement for Comp B is Picatinny Arsenal Explosive 21 (“PAX-21”). PAX-21, which is made of dinitroanisole (“DNAN”), ammonium perchlorate (“AP”), RDX, and n-methyl-4-nitroaniline (“MNA”), mimics the performance of Comp B.
When explosive compositions including RDX and TNT are heated to a temperature greater than approximately 392° F.–482° F., the explosive compositions begin a rapid, exothermic decomposition and begin to cook off. Cook-off is a hazard that affects design, testing, transportation, and storage of explosive compositions used in ordnance and occurs when the explosive composition is exposed to a source of heat. If the explosive composition is unconfined, the explosive composition ignites and burns when heated. A temperature necessary to ignite the explosive composition depends on the nature of the explosive composition. However, when stored in a confinement, such as a case, the explosive composition produces a violent reaction or explosion when heated. The explosion produces fragments of the confinement (shrapnel), which causes damage to any facilities and personnel in the vicinity of the munitions. A slow cook-off occurs when the explosive composition is exposed to indirect heat, such as when the explosive composition is stored in a facility that is adjacent to a fire. A fast cook-off situation occurs when the explosive composition is directly exposed to heat, such as when the fire is in a facility used to store the explosive composition. Explosive compositions that are insensitive to slow cook-off situations are highly desirable for military applications because they have increased safety in transportation and storage. To determine the explosive composition's tendency to cook off, the explosive composition is subjected to a gradual increase in temperature until a reaction occurs.
While PAX-21 provides a comparable melt/pour process, uses nontoxic ingredients, is less shock sensitive and shows lower sensitivity than Comp B, neither Comp B nor PAX-21 pass a slow cook-off test. In a slow cook-off test, the explosive composition is slowly heated until it ignites, explodes, decomposes, or in some other way reacts to the heat. The slow cook-off test models the temperature increase experienced by the explosive composition if it were adjacent to a burning container or storage facility. When PAX-21 or Comp B is in a slow cook-off situation, it will typically burn very quickly, unless it is confined. However, when the explosive composition is confined, the explosive composition explodes violently, sending pieces of the confinement shrapnel flying.
For the explosive composition to pass the NATO Insensitive Munitions Information Center's (“NIMIC”) slow cook-off test, it must experience a Type V Response (burning) or better. This response type allows an energetic material in the explosive composition to ignite and burn without propulsion. In addition, the case used to confine the explosive composition must split non-violently, if at all. The internal pressure of the reaction is allowed to dislodge the case cover, but debris may be thrown no more than 15 meters, and that debris should be unlikely to cause a fatality. To be an insensitive munition (“IM”), the munition must minimize the probability of being inadvertently initiated and provide reduced severity of collateral damage to facilities and personnel when subjected to unintentional stimuli. For instance, the IM should burn when exposed to fast or slow heating and should not detonate when another munition located nearby detonates.
To reduce the violence of the explosion in a slow cook-off situation, cook-off resistant explosive compositions have been disclosed. In U.S. Pat. No. 4,394,197 to Kabik et al., a cook-off resistant booster explosive composition is disclosed. The explosive composition includes a mixture of 1,3,5-triamino-2,4,6-trinitrobenzene (“TATB”), RDX or cyclotetramethylenetetranitramine (“HMX”), and a binder, such as polytetrafluoroethylene. In addition, insulating layers that prevent or slow down the transfer of heat to the explosive composition have been developed. U.S. Pat. No. 4,137,849 to Hontgas et al. discloses a liner formed from a polyvinyl chloride resin-based plastisol and s-trithiane. The liner separates the explosive composition from the confinement to prevent heat from transferring between the two. U.S. Pat. No. 5,054,399 to Bilek et al. discloses a shock attenuation liner formed from layers of material of inwardly decreasing acoustic impedance. An attenuation barnier layer is formed by arranging an explosive composition in order of outwardly decreasing detonation sensitivity.
In addition to using cook-off resistant explosive compositions and insulating layers to provide resistance to cook-off, changes to the design of the case or confinement have been proposed. The confinement includes vents to allow gases to be released as they are generated, reducing the buildup of pressure inside the confinement. The vents enable the explosive composition to burn rather then explode.
In U.S. Pat. No. 5,959,235 to Wagstaff, a device used to ignite a propellant before it detonates is disclosed. The device uses two metals, a first metal that melts at a temperature below a detonation temperature of the propellant and a second metal that reacts with the first metal to produce an exothermic reaction that causes the propellant to burn. The first metal is sodium and the second metal is an alloy of bismuth, lead, tin, cadmium, tellurium, and/or antimony. The device is incorporated into an igniter or is located inside the propellant.
An autoigniting composition used in an automobile occupant restraint system is disclosed in U.S. Pat. No. 5,084,118 to Poole. The autoigniting composition includes an alkali metal or alkaline earth metal chlorate, 5-aminotetrazole, and 2,4-dinitrophenylhydrazine. The autoigniting composition is used in an aluminum pressure vessel to contain the composition and gases produced during ignition of the composition.
It would be desirable to provide an explosive composition that is resistant to slow cook-off. Specifically, it would be desirable to produce a Comp B or replacement for Comp B that mitigates violence and passes NIMIC's slow cook-off test.